MDI is one of main chemicals in polyurethane industry. It is a well known process in polyurethane industry that a condensation reaction of aniline with formaldehyde in the presence of an aqueous acid catalyst (generally hydrochloric acid) is conducted to form polymethylene-polyphenyl-polyamine, and then a phosgenation reaction of polymethylene-polyphenyl-polyamine with phosgene is conducted to form MDI monomer and polymeric MDI.
One of the key steps in the preparation of MDI is that polymethylene-polyphenyl-polyamine hydrochloride obtained from the condensation reaction of aniline with formaldehyde in the presence of hydrochloric acid catalyst is neutralized with caustic soda, and then the polyamine layer is separated from the salt water layer, washed with process water and refined to form polyamine. The above neutralization and washing processes produce a great amount of waste brine, which contains organic compounds such as aniline, diaminodiphenylmethane, and polyamine. The waste brine has to be treated by extraction and steam stripping, and examined to ensure the pollutant content meets the requirements before it is discharged. The extractant used to treat the waste brine in the manufacture process of MDI is usually selected from toluene, xylene, chlorobenzene, dichlorobenzene or aniline and the like, and the extraction device is usually selected from a mixer-settler extraction column, an extraction column or a Graesser contactor and the like. The disadvantages of abovementioned extraction process are large device volume, big investment, low extraction efficiency, long extraction equilibrium time, easily blocking up of column plates or fillers by salt muds and so on, which result in that the level of organic amines in the discharged waste brine is relatively high and easily fluctuated.
Generally, the waste brine in the manufacture process of MDI, after treated by regular extraction and steam stripping, contains about 14%-18% NaCl, and its pH is usually in a range of about 12-14, TOC is about 30-50 ppm and TN is about 3-5 ppm. According to the requirements in the manufacture process of ion membrane caustic soda, the above-described waste brine only meets the discharge limits rather than being used directly as a feedstock due to its high level of organic amines. Considering the great amount of waste brine discharged in the manufacture process of MDI, it is a considerable waste of water resource and salts. From the view of sustainable development and circular economy, it highly desirable that the great amount of waste brine is deeply treated to make the organic content meet the requirements of the manufacture process of ion membrane caustic soda, and then it can be used as a feedstock of ion membrane caustic soda plant to produce chlorine, caustic soda, hydrochloric acid and hydrogen and the like which are basic materials in the manufacture of MDI. It would achieve the circular use of materials in MDI industry chain by recovering resources such as sodium chloride and water from the waster salt water in the manufacture of MDI.
Chinese Patent ZL200710013817.2 describes a continuous process of extracting polymethylene polyphenyl polyamine from its salt water solution. The disadvantage of the method is that the waste brine contains a relatively high level of organic compounds (high TOC and TN values) without following deep treatments and only applies strictly to a diaphragm electrolysis in the manufacture of caustic soda from the view of safety. In a long term view, the diaphragm electrolysis process is restricted by policies due to its high energy consumption, and it would be replaced by ion membrane electrolysis which has stricter requirements on the organic content of the salt water.
Chinese Patent ZL200710138065.2 describes a method of deep treatment of waste brine in MDI manufacture process. The disadvantages of the method are that the stripping tower consumes too much steam and energy because only one regular extraction treatment is conducted, and the deep treatment consumes a considerable amount of oxidants and absorbents due to the high level of organic content in the waster salt water prior to the deep treatment, which causes a high cost.